(1) Field of the Invention
The present invention relates to an image forming apparatus, and in particular to a technology for performing a temperature control in a fixing device of the image forming apparatus.
(2) Description of the Related Art
In an image forming apparatus of the photoelectric method, after a charger electrically charges the surface of a photosensitive drum in a unified manner, it forms an electrostatic latent image on the photosensitive drum by emitting a laser beam to expose-scan the surface of the charged photosensitive drum, and develops the electrostatic latent image by causing a developing unit to supply toner thereto. The toner image formed on the surface of the photosensitive drum is transferred, via an intermediate transfer belt or directly, onto a recording sheet. Then the recording sheet with the toner image is heated and pressured by a fixing device and the toner image is fixed thermally onto the surface of the recording sheet.
Meanwhile, the temperature of a heating rotating member required for the thermal fixing (fixable temperature) is generally high, for example, 200° C. Especially when the heating rotating member is in the shape of a roller, the thermal capacity is large, and thus requires a lot of time for the warm-up. For this reason, in many apparatuses, the heating rotating member is always kept to be at a temperature close to the fixable temperature, so that a print process can be started as soon as a print job is received (hereinafter, the state is referred to as a “wait state”, and the temperature maintained in the wait state is referred to as a “wait temperature”. Typically, the wait temperature is set to a temperature that is lower than the fixable temperature by approximately 20° C. to 40° C. so that the fixable temperature can be reached in approximately 10 seconds). Note that in the present specification, “fixable temperature” means a target value of the control for adjusting the surface temperature of the heating rotating member to be within a range of temperature that is required for the toner to be fixed on the recording sheet. Thus in the actual control, slight ripples are generated around the target temperature.
FIG. 24A is a graph showing the change in the temperature (hereinafter referred to as a “fixing unit temperature”) T of the heating rotating member during a conventional temperature control of the fixing device, wherein the horizontal axis represents an elapse time t from the activation of the apparatus, and the vertical axis represents the fixing unit temperature T.
After the apparatus is powered on (t=0), the power is supplied continuously to the heater of the fixing device (indicated by “Heater ON” in the graph), and the warm-up is started. When the fixing unit temperature T reaches the wait temperature 170° C., the temperature control is switched to a control in which the power supply to the heater is switched ON/OFF intermittently to maintain the wait temperature. Upon receiving a print job and an instruction to start printing, the apparatus continuously supplies the power to the heater (“Heater ON”) to increase the fixing unit temperature T straight to 200° C. in approximately 10 seconds, then starts the printing, and after the printing is over, stops the power supply to the heater to decrease the fixing unit temperature T. When the fixing unit temperature T reaches the wait temperature 170° C., the apparatus performs the control to switch ON/OFF the heater intermittently to maintain the wait temperature.
Meanwhile, most of the power consumed by the image forming apparatus is the power required for heating the heating rotating member in the fixing device. Accordingly, for the sake of energy conservation, many power saving technologies have been proposed conventionally.
For example, Japanese Patent Application Publication No. 2007-65597 discloses a structure of an image forming apparatus that has an operation mode including a recording mode and a non-recording mode, wherein the recording mode causes the fixing device to execute a recording operation for fixing an image onto a recording sheet, and the non-recording mode is performed to execute a non-recording operation other than the recording operation. While the operation mode is set to the recording mode, the temperature of the heater in the fixing device is adjusted to be the fixable temperature, and while the operation mode is set to the non-recording mode (during the time period α), the temperature adjustment of the heater is stopped and the power is not supplied (“Heater OFF”), as shown in FIG. 24B.
With the above structure, while the operation mode is set to the recording mode, the recording operation can be started immediately, and while the operation mode is set to the non-recording mode, the power supply to the heater of the fixing device is stopped, thereby preventing wasteful consumption of the power.
However, according to the above conventional technology, the control is made to stop the power supply to the heater for all the period while the non-recording mode is executed, and after the mode is switched to the recording mode, the heater is powered ON. Accordingly, the user has to wait for a time period “tv” required for the warm-up until the printing process is started. This is inconvenience for the user.